In a conventional compression ignition (CI) engine a single piston is slidably disposed in a cylinder. The piston moves in the cylinder between a top dead center (TDC) position where the crown of the piston is closest to the closed end of the cylinder, and a bottom dead center (BDC) position where the crown is furthest from the closed end. Air introduced into the cylinder is compressed by the piston as it moves toward TDC during its compression stroke. Compression of the air raises its temperature. Liquid fuel is injected into the resulting hot air at a time near the piston reaching the top of its compression stroke. The elevated temperature of the compressed air causes autoignition of the fuel whereby the fuel self-ignites and burns, releasing energy and driving the piston toward BDC in a power stroke.
In an opposed piston, two-stroke, compression ignition engine, two pistons are slidably disposed crown-to-crown in the bore of a cylinder having inlet and exhaust ports near BDC of each piston, with the pistons serving as the valves for the ports. The pistons move coaxially in the cylinder, toward and away from each other, between their TDC and BDC positions. Air introduced into the cylinder is compressed by the pistons as they move toward each other to their respective TDC positions during a compression stroke. The opposed piston CI engine typically has a liquid fuel injector mounted to the cylinder at a location near the TDC position of the piston crowns, usually at, or very near, the longitudinal center of the cylinder. The injected fuel mixes with the compressed air and the air/fuel mixture autoignites, driving the pistons away from each other in a power stroke toward their BDC positions. One such opposed piston engine is disclosed in the referenced '707 patent application.
Compression ignition engines are characterized by a number of undesirable features. One drawback is that the fuel injector is positioned so that injection occurs at or near TDC of the pistons, leaving little time for the injected fuel to vaporize and mix with the compressed air before autoignition occurs. The heterogeneous mixture of air and fuel burns unevenly. Also, some of the injected fuel collects on the surface of the cylinder bore and remains in the orifices of the injector where it fails to burn at all. The result is production of NOx (oxides of nitrogen) and particulate matter (smoke). Further, location of the fuel injection site on the cylinder near piston TDC exposes the injector to the highest pressures and temperatures that occur in the cylinder. The extreme temperatures of combustion can cause fuel trapped in the orifices of the injector to boil during the power stroke, which produces pollutants such as carbon monoxide (CO), unburned hydrocarbons, and soot. Over a period of time the extreme heat can cause carbon to accumulate at the orifices of the injector, thereby interfering with the fuel injection pattern and producing an uneven burn that increases NOx, hydrocarbon and particulate emissions during the power cycle.
In an effort to overcome the problems stemming from heterogeneity of the air/fuel mixture in CI engines, homogeneous charge compression ignition (HCCI) engines have been proposed. The proposals are typically for single piston configurations. In this regard, HCCI operation is the process wherein a homogeneous mixture of air and liquid fuel is ignited by compression of the mixture. In HCCI engines, fuel is injected into the cylinder early in the compression stroke, well before the temperature of the air has reached a level that could initiate autoignition. Early injection assists the fuel to evaporate and disburse throughout the air in the cylinder, with the goal of forming a substantially uniform air/fuel mixture (the homogeneous charge) that is further compressed until autoignition occurs. The amount of fuel injected is controlled in order to provide a lean air/fuel mixture and to control the combustion process in order to yield a significant reduction in NOx and particulate emissions as compared with conventional CI engines.
HCCI combustion was introduced as an alternative to spark ignition (SI) for two cycle internal combustion engines in 1979. When Federal EPA standards requiring drastic reductions in internal combustion engine emission pollutants were introduced in the early 1990's, research and development in HCCI technology saw a dramatic increase. “Over the last seven years, numerous studies have been reported to explore the potential of this technology and many innovative strategies for mixture preparation, combustion control, load extension and emission reduction have been proposed and developed by automotive companies, diesel engine manufacturers, component suppliers, and research institutions.” [Homogeneous Charge Compression Ignition (HCCI) Engines, Key Research and Development Issues, SAE Order No. PT-94, 2003].
Although HCCI does provide potential benefits such as fuel efficiency, reduced NOx and lower particulate emissions, this combustion mode also poses problems that the industry and R&D in general have not yet solved. Among the drawbacks of the HCCI engine are high hydrocarbon and CO emissions during certain engine operating conditions, difficulty in controlling combustion timing, and the inability to operate over a broad load range, particularly at power levels in excess of 30% of maximum power.
During low load operation of the HCCI engine, the injected fuel is incompletely burned, which produces significantly higher levels of hydrocarbon and CO emissions than a spark ignition (SI) engine. At high load operation of the HCCI engine it is extremely difficult to control autoignition due to the lack of control of the stoichiometry of the air/fuel components responsible for ignition; early combustion results, producing high engine noise and possible engine damage. For these and other reasons, a multimode HCCI engine has been proposed. During the middle of the operating load range the multimode engine operates as a HCCI engine and, during low load and high load conditions, as a conventional CI or SI engine. The multimode design demands a complex fuel injection system or requires two different fuels; both solutions are cumbersome and add to manufacturing and operation costs.
Thus, a single-mode homogeneous charge engine of the compression ignition type, capable of operating over a broad load operating regime while producing low levels of emission pollutants including NOx, CO, hydrocarbons and particulates, is an extremely desirable objective.